The present invention relates to a method and apparatus for pressure or die casting of metals, for example aluminum, whereby the pressure casting dies are evacuated. Heretofore several steps have been taken in connection with pressure die casting to improve the quality of the cast work pieces particularly to improve the structure or texture of such work pieces, especially to avoid or at least diminish air and gas inclusions.
Thus, it is for example known to connected the hollow spaces of the dies and of the filling chamber to a vacuum pump for evacuating these spaces. For this purpose, several steps are necessary to make the dies or casting molds and the casting machines compatible for the performance of such methods. For example, it is necessary to provide special suction channels having predetermined lengths. Further, it is necessary to assure a timely switch-off between the vacuum conduit and the hollow spaces as well as the channel system of the mold. This was accomplished heretofore by means of valves and control systems for such valves. However, such switching-off is a source of trouble and causes problems in the operation of such prior art casting machines.
Basically, all prior art pressure die casting machines operate on the principle that the filling opening of the filling chamber of the pressure casting machine is closed after filling of the molten metal is completed, said closing being performed by the feed advance of the pressure applying piston. Simultaneously, with such closing the evacuation of the die and the filling chamber is switched on. It is a substantial disadvantage of this type of operation that for evacuation of the total volume of all the hollow spaces, that is of the filling chamber, of the dies, and the suction channels, there is very little time available, namely the time during which the pressure applying piston advances to the point of filling the die or mold.
Yet another disadvantage of prior art devices is seen in that their operational safety is quite different in different known devices. However, in all prior art devices there is the danger that as a result of a failure of the mentioned control mechanisms, it is possible that the switching-off of the vacuum does not take place on time whereby molten metal may enter into the control devices, whereby the latter may be destroyed or at least damaged.
Another problem with prior art devices is the possibility of spilling of the molten metal. In this connection it has been proposed to control the feed advance of the pressure piston to move the latter first slowly and then with an increasing speed so that such spilling of molten metal in the horizontally arranged filling chamber is avoided. A further purpose of such piston advance control is to avoid the production of porous cast work pieces by preventing the formation of a metal air mixture.
Another drawback of prior art methods for pressure or die casting is seen in that the rather expensive control mechanisms and additional steps are geared to particular types of machines and cannot be employed where it is desired to subsequently adapt molds and machines to pressure casting procedures.
Another requirement to be met in pressure die casting is the avoidance of oxide formations on the surface of the liquid or molten metal during the transporting, the filling and during the casting proper. This may be accomplished, for example, by covering the molten metal surface with an inert gas during the ladling, dosing and casting operations. However, for this purpose again expensive equipment is necessary. Besides, substantial quantities of inert gas are also required.
In connection with the so called cold chamber pressure casting method, the molten metal is supplied by means of a ladle which removes the molten metal from a crucible furnace into the filling opening of the die casting machine. A number of mechanical ladling mechanisms are known in the art capable of performing or controlling the ladling operation by mechanical or automatic means.
Aside of the casting devices operating with mechanical ladling mechanisms, there are devices known in which the molten metal is sucked into the filling chamber by evacuating the pressure casting die or mold as well as the filling chamber through a riser pipe immersed into the molten metal. These prior art devices are known under their respective names Nelmor, Morton and others. However, the known devices did not catch any substantial market, because of the requirement that the liquid metal supply must be located at a level lower than the level of the filling chamber of the pressure casting machine. Another problem of these known machines is seen in the dosing or proportioning of the metal supply. In addition, the entire filling pipe is exposed to the free atmosphere upon completion of the respective casting operation. This causes oxidation problems internally of the die or mold as well as relative to the metal to be cast whereby the quality of the cast work pieces is influenced in a disadvantageous manner.